This invention relates generally to operational amplifiers, and more particularly, to an operational amplifier integrated circuit having an all NPN output stage and including passive current limiting circuitry.
An operational amplifier including an output stage comprised of all NPN transistors is shown and described in U.S. Pat. No. 4,403,200 issued Sept. 6, 1983 and entitled output stage for operational amplifier. In this circuit, all of the NPN transistors are activated in order to sink current. It is therefore necessary to provide circuitry which prevents these transistors from being damaged when the output is shorted to one of the supply rails. A solution used in the past involves sensing the short circuit current flowing into the output terminal and utilizing this current in a feedback loop so as to deprive base current flowing into the second stage and output stage transistors. Specifically, a sense resistor R.sub.SC senses the short circuit current flowing into the output node. When the voltage drop across this sense resistor reaches the base emitter voltage of, for example a PNP transistor, the transistor turns on causing current to flow through its emitter-collector path to a diode. An additional transistor is coupled in a current mirror configuration with the diode and has a collector coupled to the base of the second stage follower transistor. Thus, the current flowing through the diode is mirrored at the collector of the additional transistor thus diverting base current from the second stage.
Unfortunately, the two transistors, diode and interconnect metal consume significant die area. Furthermore, the resulting circuitry represents a feedback loop which under certain conditions could oscillate. This requires the inclusion of additional compensation circuitry to render the current limit circuit frequency stable.